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A new Mg2Si-production process promises to advance thermoelectric
devices.
The Seebeck effect - the direct conversion of a temperature difference to electricity - is responsible for the operation of thermocouples. But taking advantage of this phenomenon to make electricity from waste heat in so-called
thermoelectric (TE) devices has been hindered by the high cost or toxicity of
common TE materials, such as leadtellurium or cobalt-antimony. And up to now,
production of the non-toxic alternative material, magnesium silicide (Mg2Si),
has been hindered by difficulties in growing Mg2Si crystals. This situation
may change, thanks to a new process developed by Tsutomu lida, associate
professor at the Dept. of Materials Science & Technology, Tokyo University of Science (Japan;
edlinks.che.com/7370-564).
To make the Mg2Si, a stoichiometric mixture of granular Mg
(99.95%) and powdered Si (99.99999%) is first ground into a powder in a
controlled environment. A dopant, such as bismuth (for n-type TE electrodes) or
silver (for p-type), is added to the powder, which is then sintered under a
mixture of argon and hydrogen at 0.08 MPa pressure and a temperature 20K above
the melting point of Mg2Si. Ingots are then sliced and polished to
make 1-2-mm thick wafers, which are then made into stable, durable electrodes by
plasma-deposition ofNi onto the surface.
The Mg2Si electrodes operate over temperature differences of
about 500°C; with a 10-ohm load, the electrodes are capable of generating up to 2 rnA at 48 m V. A prototype device under
construction, consisting of 12 wafers, each with an area of (7 X 7) cm2,
is expected to generate 118 W of power from a temperature difference of 250°C,
based on tests conducted on small pieces in the laboratory, which generated 0.2
W/cm2. lida believes the Mg2Si can be applied for
generating electricity from the waste heat in the fluegas of power plants and
incinerators, and the exhaust of automobiles.
Mg2Si ingots are being produced
semicommercially
(5-25 kg/mo) by Union Materials Inc. (Ibaraki, Japan), and the material is being marketed by
Showa KDE Co. (Tokyo)
since January. The current price is roughly $900 per 100 grams, and future
commercial mass production is expected to cut the cost roughly in half, says
lida.
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